1. Technical Field
This invention relates generally to recorders for forming images on strip recording media such as for example silver or non-silver, light sensitive film and the like; and more specifically to such recorders having a rotating drum which carries the media past exposing optics.
2. Background Art
Rolls of strip film has been used for years as the preferred medium for photographically recording digital images. Cathode ray tubes (CRT's) are commonly used as the exposing mechanism used for low-resolution imagery because CRT's provided a simple, reliable method for modulating a light beam which is scanned across a stationary section of film. While this technology continues to be used for low-resolution imagery, there exists a number of physical factors of the CRT method that limits its performance for high quality digital images.
The CRT has a thin coating of phosphor that emits photons when irradiated by an electron beam. Because of the high external pressure on the CRT, the face plate thickness must be at least approximately 1 cm. thick. This thickness causes multiple surface reflections of the photons which reflect back to the phosphor causing secondary emissions. This effect manifests as a halo around the CRT beam which consequently causes a "flare" effect in the exposed image. The flare problem limits the maximum amount of light to the film, thereby limiting the exposure range of the CRT system.
Other drawbacks of CRT systems include changes in the beam shape as the beam is deflected across the face plate, graininess of the phosphor which is imaged onto the film, and off-axis optical imaging resulting in reduced image sharpness at the periphery of the image. Dust attraction due to the high electrostatic field at the surface of the face plate also presents a problem.
The CRT exposure problems of poor exposure range and image sharpness are overcome by the use of electro-optic modulators as the primary means to vary the intensity of collimated light beams that are scanned across the surface of a light sensitive recording medium. Commonly assigned U.S. Pat. No. 4,631,551 discloses such a color imaging apparatus wherein the material used in the electro-optic modulators is lanthanum-doped lead zirconate titanate (PLZT). This technology is best suited for rotating drum type film recorders.
Rotating drum recorders are well known. Conventionally, the light sensitive recording medium is wrapped about the outer cylindrical surface of the drum, as shown in FIG. 1, and is exposed by a light source external to the drum. A modulated light beam from a focusing lens 22 passes through an aperture 24 and is collected by a lens 28. Lens 28 focuses this light beam as a spot on the surface of a light sensitive member disposed on a drum 30. Drum 30 is translated a lead screw 31 which is driven by a stepper motor 31a controlled by a microprocessor 33.
Conventional rotating drum recorders must be loaded in the dark or in "safe light" conditions. The outer cylindrical surface of the drum can be very accurately produced, but the varying thickness of the media will adversely effect the focus of the system, since it is the back side of the medial that is controlled by the drum surface, not the front, light sensitive side.